This invention relates generally to a method and apparatus for motor damping. In particular, detecting the integrity of a damping network for a motor to ensure its operation.
Direct current (DC) brushless motors are synchronous machines having a permanently magnetized rotor free to rotate within fixed stator coils. Phased alternating currents passing through the stator coils generate a magnetic field that rotates the rotor. In certain applications using DC brushless motors, such as electric power steering systems, it may be necessary to rapidly stop the rotation of the motor. In order to achieve a rapid stopping or damping, the kinetic energy of the rotating motor shaft must be quickly dissipated. One simple method of motor braking uses a mechanical brake in which friction, such as between brake pads and a rotating surface, dissipates the kinetic energy of the rotor as heat. Alternatively, dynamic braking takes advantage of the fact that a coasting DC motor acts like an electrical generator. In dynamic braking, a resistance is shunted across the stator windings, thereby allowing the energy of the coasting rotor to be converted to electrical energy and dissipated within the resistance as heat.
Electromagnetic relays are often used to provide a shorting capability for one or more stator windings of a DC brushless motor. A typical electromagnetic relay features an electromagnet which, when energized, attracts a moveable iron member to the core of the electromagnet. In turn, the moveable member may separate a moveable electrical contact from a stationary contact (or join a moveable contact to a stationary contact). An electromagnetic relay, being an electromechanical device, may be susceptible to wear and tear and hence may experience degraded performance.
In addition, arcing during the switching of the relay contacts may cause degradation in the conductive properties of the contacts through the deposition of carbon or the creation of oxides at the point where the contacts meet. Carbon deposits and oxides can increase the contact resistance and affect the damping operation of the relay, if used in such an application. Therefore, what is needed in the art is a technique that provides assurance of the operability of the damping network at low cost.